The measurement of the quantity of liquid fuel for aircraft tanks is currently done by means of electrical equipment whose functioning principle is based on the induction of an electric current in a circuit with a variable permeability capacitor (the permeability of the dielectric of the capacitor is a function of the fuel level). The measuring equipment used in this type of arrangement is known as a gauge. Its design consists of two concentric cylinders each connected to an electric pole, between which an electric field is generated. According to the classical theory of capacitors, the capacitance of a capacitor is a function of the permeability of the dielectric existing between the surfaces of the two cylinders. This dielectric is in turn a function of the level of fuel which the tank has at any moment, in other words, when the gauge is not entirely submerged in the fuel the dielectric will consist of a percentage of air corresponding to the non-submerged part, and a percentage of fuel corresponding to the submerged part. In this way, a defined measurement of voltage will be obtained. The variation in the voltage (and the later treatment of each of the signals obtained from each equipment) in each of the gauges distributed throughout the entire tank reproduces the volume of liquid in it.
One of the main drawbacks of this type of configuration for measuring fuel is the fact of having to introduce an electric current inside a fuel tank containing inflammable fuel, even though these currents are of low intensity and low voltage, along with the increase in the complexity of assembling the installation. For this reason it is necessary to protect the electric cables to which the gauges are attached, and also to earth the bundles of electric cables which need to be included inside the tank due to the introduction inside of a considerable number of gauges, since they are installed in pairs in order to provide redundancy.
Moreover, it can be pointed out that it is known in the state of the art that the fuel contained in a tank can be measured by means of optic fibre cables running along the inside of the tank, these cables being connected to some measuring equipment for the volume of fuel which uses a light transmitter which injects light from one of the ends of the optic fibre, so that in the other end the beam of light impinges on the liquid contained in the tank. The signal which is going to be reflected has a wavelength that depends on the medium the light is propagating in, in other words, if the end of the fibre emitting the light is covered with fuel (fibre wetted by liquid) the propagation in this medium gives rise to a transmitted wave having certain characteristics, while if the light is propagated through air (dry fibre), then a different signal from the previous one is obtained in a light receptor which is connected to a signal processor, which obtains the measurement of the volume of liquid contained in the tank.
The use of optic fibre with the corresponding measuring equipment has not been applied for measuring the volume of fuel in the tanks of aircraft.